Most separation of commercial sugar crystals from molasses occurs in centrifugal separators. However, this technique has many disadvantages in that, in order to develop the required centrifugal force, the baskets on which the screens are mounted must be rotated at relatively high speeds (usually from about 1,000 to 1,800 rpm). Operation thus requires considerable power, typically in the neighborhood of 50 to 100 horsepower, further necessitating careful dynamic balance of the rotating parts and sturdy construction of the entire system. A further difficulty with centrifugal separators is that they operate in a batch mode and thus have to be stopped for admitting the massecuite and then again for discharging the sugar crystals, thereby resulting in a loss of time. There have been attempts to design continuous mode centrifugal separators, but they have not been commercially successful due to defective separation, often accompanied by sugar crystal breakage.
Crystals from the centrifugal separator are typically characterized by a moisture content in the range 0.5-0.7%, while the end product is required to have a moisture content less than about 0.2%. Thus the crystals are sent to dryers. These dryers are rather large drum devices in which the sugar crystals to be dried are introduced and cascaded within the drum as a stream of hot air is directed through the drum to evaporate the moisture. Unfortunately, the cascading action causes considerable crystal breakage and actually causes a fraction of the sugar crystals to be transformed into a powdered form. This presents serious dangers of explosion and thus necessitates costly precautionary procedures.
Continuous separation systems are known. In particular, U.S. Pat. No. 701,687 to Desaulles discloses a rotary screen device wherein a fixed inlet chute deposits massecuite on a rotating screen while a screw auger removes sugar crystals from the screen after substantially an entire rotation. Removal of the molasses is effected by a vacuum tank rotating along with the screen that serves to draw the liquid through the screen. While this system is capable of operating in a continuous mode, it nevertheless subjects the sugar crystals to excessive mechanical handling with the result of crystal breakage. Additionally, it should be noted that the crystals still require further handling such as purging, drying, etc., and the attendant costs and dangers associated therewith.
An alternate approach to the continuous recovery of commercial sugar crystals and molasses has been tried, and employs a filtering belt and a casing under vacuum. During operation, the massecuite is fed continuously on top of the belt and the molasses recovered into the casing by the suction action of the vacuum. The remaining sugar crystals are eventually scraped off and removed from the belt and sent to a dryer. The molasses is transferred from the casing to a degassifying chamber before being pumped from the system. Apparently, problems have been encountered in maintaining the required vacuum within the casing due to cracks in the sugar layer arising from the inability to keep a belt truly flat and level.
Accordingly, there is presented a need for a continuous separation system of simple design and operation which avoids the problem of crystal breakage.